Ray tracing for Jupiter’s icy moon ionospheric occultation of Jovian auroral radio sources

The ionospheres of Jupiter’s icy moons have been observed by in situ plasma measurements and radio occultation. However, their spatial structures have not yet been fully characterized. To address this issue, we developed a new ray tracing method for modeling the radio occultation of the ionospheres using Jovian auroral radio sources. Applying our method to radio observations with the Galileo spacecraft, we derived the electron density of the ionosphere of Ganymede and Callisto. For Ganymede’s ionosphere, we found that the maximum electron density on the surface was 150 cm^-3 in the open magnetic field line regions and 12.5 cm^-3 in the closed magnetic field line region during the Galileo Ganymede 01 flyby. The difference in the electron density distribution was correlated with the accessibility of Jovian magnetospheric plasma to the atmosphere and surface of the moons. These results indicated that electron impact ionization of the Ganymede exosphere and sputtering of the surface water ice were effective for the producing Ganymede’s ionosphere. For Callisto’s ionosphere, we found that the densities were 350 cm^-3 and 12.5 cm^-3 on the night-side hemisphere during Callisto 09 and 30 flybys, respectively. These results combined with previous observations indicated that atmospheric production through sublimation controlled the ionospheric density of Callisto. This method is also applicable to upcoming Jovian radio observation data from the Jupiter Icy Moon Explorer, JUICE.